1. Field of the Invention
The present invention relates to a rolling roll for manufacturing seamless steel pipe, and a method for making the roll. More specifically, the present invention relates to technology, in manufacturing seamless steel pipe by means of the Mannesmann system, for improving wear resistance of a roll used in a rolling mill, and its heat crack resistance, biting properties, preventing surface roughness and the like, all by combining special ingredients of the roll and the metallurgical structure of the roll.
2. Description of the Prior Art
In manufacturing seamless steel pipe by the use of the Mannesmann system, the biting property of the roll upon the pipe is an essential factor in order to achieve advantageous rotary forging. But it is not enough simply to apply a little soft material to the roll to be used to improve its biting upon the steel pipe, because of the resulting loss of wear resistance.
To reduce the manufacturing cost of the seamless steel pipe, it is very important to extend the working lives of rolls needed to be used. Furthermore, if a stainless type steel pipe is manufactured with a roll that has poor wear resistance, it is difficult to ensure the surface quality of the inner and outer surfaces of the steel pipe. Accordingly, a roll having a so-called Adamite type material has been manufactured by centrifugal casting. However, notwithstanding its wear resistance, the roll is ineffective on seamless steel pipe because the problem of its biting property remains.
Referring to FIG. 4 of the drawings, a piercer roll 1 is so arranged that the roll is inclined at a stand. The piercer roll 1 is different from a normal roll and comprises three portions: (a) an introducing portion (entrance) 2 for the material 5 to be rolled, (b) a contributing portion 3 to the rolling and (c) a delivery portion 4 of the material 5.
Since respective portions of the rolls have different functions, the characteristic necessary for the roll material is that each portion of the same roll shall differ from the others. That is, at the entrance 2 for the material 5 to be rolled, the biting property of the roll upon the material to be rolled is important. On the surface of the entrance 2, some surface roughness must be maintained to provide friction. In addition, at the contributing portion 3 at a center portion of the roll, the roll material needs wear resistance and needs to prevent excessive surface roughness. At the delivery portion 4, the material 5 to be rolled must be stably held by the roll; accordingly, some surface roughness and wear resistance are required there. It is difficult for any current technique to satisfy such a difficult combination of requirements.
Moreover, in such a piercer roll 1 significant work hardness develops near the entrance 2, and considerable friction arises at the roll center portion 3, and these influences must be overcome. To improve wear resistance, at least a portion of the roll material must have a high carbon content (for example, 1.9 wt %). While the surface roughness of the roll surface is thereby temporarily improved, another problem arises. During working, poor biting due to work hardness at the entrance 2 causes slippage of the material 5 to be rolled. This results in seizing and damage. More specifically, in case of excessively poor biting, the material 5 to be rolled cannot be bitten, sometimes entirely preventing rolling. When the roll is made of a tool steel material which has a lower carbon content than the roll material, such as 1 wt %, which has a relatively high hardness, excellent wear resistance is obtained, and biting effectiveness is also maintained at the entrance side portion 2. However, this causes deep heat cracking to occur at the contributing portion 3, which causes the roll surface portion to break off.
Japanese Patent Publication No. 44-17022/1969 and Japanese Patent Publication No. 48-7180/1973 are of interest. They provide a method for preventing heat cracking and break-off of the roll surface wherein, in manufacturing the roll, roll toughness is far advanced by hot-forging the roll after forging. However, the rolls manufactured by this technique have high surface hardness but poor biting properties.